Reference to background art herein is not to be construed as an admission that such art constitutes common general knowledge.
Precision machine guidance is becoming increasingly common in many industries, primarily due to productivity improvements that can be obtained with the assistance of accurate machine control systems. In civil construction and mining machinery such as excavators, dozers, and graders can all benefit from precision machine guidance and control.
Machines equipped with precision guidance require drastically reduced interaction with surveyors on site, as the precision guidance equipment is able to compute the position of a working edge (e.g. of an excavator bucket or a dozer blade) with respect to a reference, such as a string line. Where the machine is also equipped with a precision positioning system such as a Global Navigation Satellite Systems (GNSS) Real Time Kinematic (RTK) system, or an optical instrument such as a total station, the need for a surveyor working on site is reduced significantly or, in some cases, able to be eliminated entirely.
Real-time feedback of the position of the working edge also allows an operator of a machine to be more efficient through fast identification of material to be moved (e.g. cut or filled) and in reducing re-work required to obtain a desired outcome. Even more advanced applications that rely on precise and timely positioning include semi-automatic or fully automatic control of the working equipment, which further enhances the speed at which the machines can work and reduces the training and experience required by an operator to control the machinery.
Civil construction and mining sites often require centimeter, or even millimeter, precision. Such high precision needs directly impacts the performance requirements of guidance equipment and constant improvements are sought to increase accuracy at a reasonable cost.
Many successful machine guidance solutions on the market, such as the Leica Geosystems iCON Excavate iXE3 system, consists of inclinometers or accelerometers mounted to movable members of a machine, such as an arm of an excavator having a boom, stick, and bucket. The inclinometers measure a local gravity vector in the coordinate frame of the member to which it is attached, and thus it is possible to calculate the angle of that member with respect to a vertical axis. Knowledge of the angle of each movable member, together with the position and orientation (pitch, roll, and heading) of the chassis and the geometry of the machine allow the position of a working edge (e.g. the lip of an excavator bucket) to be calculated.
Current systems typically have good static performance, i.e. determining a position when there is no movement, but very poor dynamic performance, i.e. determining a position when there is movement. In typical usage, an operator has to pause periodically to allow the equipment to stabilise in order to get an accurate position measurement. Depending on the action being undertaken, several pauses may be needed. Such periods of inactivity obviously reduce efficiency as they reduce the amount of time for which equipment can be working.
While direct measurement of member positions may be measured using, for example, rotary or angle encoders or the like, such sensors are difficult to retrofit to a machine and are subject to maintenance and calibration. Accordingly, such direct measurement systems are usually not practical or commercially viable for after-market guidance systems and most systems therefore rely on inclinometers for angular measurements.
The root cause of the poor dynamic performance of an inclinometer-based measurement system is due to measurements of a gravity vector being corrupted by accelerations, shocks, and vibration caused by the movement of the machine and its movable members.
This effect is observed in the aerospace industry where a manoeuvring aircraft can induce significant errors into a measurement of the gravity vector. The solution adopted by the aerospace industry is to utilise gyroscopes, which measure the rate of change of angle with time, whose measurements are accumulated to track the angle. Since the accumulation of gyro measurements will also accumulate errors, the calculated angle is slowly steered to the measurements of the gravity vector. This complementary arrangement of sensors is known as an Attitude and Heading Reference System (AHRS).
The recognition that gyroscope measurements can help improve the dynamic performance of a machine guidance solution is slowly finding its way into products on the market. Whilst the inclusion of the gyros improves the performance of inclinometers, merely smoothing variations in the gravity vector offers suboptimal performance. That is, the accuracy of the solution is still limited by the quality of inclinometers and the associated measurement of the gravity vector, since the accumulated noise, bias, and other sensor errors from the gyroscopes eventually cause the angle to drift. Even with perfect inclinometer measurements, induced accelerations caused by movement will ultimately limit the performance of such a system.
It is therefore desirable to understand the cause and effect of accelerations induced on inclinometer measurements. If the induced accelerations can be identified, quantified, and observed, then the measurements of the gravity vector can be duly compensated.